A surface acoustic wave (SAW) device is well known as a device of application of acoustic waves. A SAW device is utilized in various circuits for processing radio signals across a frequency band ranging from 45 MHz to 2 GHz typically used for cellular phones, etc. Examples of the various circuits include transmission band-pass filters, reception band-pass filters, local oscillation filters, antenna duplexers, IF filters, and FM modulators.
In recent years, the acoustic wave devices are required to improve the characteristics such as high suppression over frequencies out of the band in use and an increase of a temperature stability due to demands of higher performances and downsizing of cellular phones. The method for improving temperature stability has been developed in which a dielectric such as SiO2 is formed on an interdigital electrode of a piezoelectric substrate. Furthermore, boundary-wave devices have been developed to downsize the devices using the boundary-wave devices. The boundary-wave device has an additional dielectric layer on the dielectric layer formed on the interdigital electrode due to trap energy between the surfaces of the additional dielectric layer and the piezoelectric substrate.
Almost of the acoustic wave devices sometimes have a common problem of variations of frequency which is one of the characteristics of the acoustic wave device and the variations of frequency are caused by manufacturing variations. To solve this problem, various methods for adjusting a frequency have been disclosed (e.g., refer to Japanese Open-laid Patent Publication No. 02-301210 and International Publication 2005-083881).
As an example, Japanese Open-laid Patent Publication No. 02-301210 has proposed the method in which the frequency is adjusted by forming a SiN film on an interdigital electrode, a reflector, and a piezoelectric substrate with, for example, plasma CVD. Alternatively, International Publication Pamphlet No. WO 2005/083881 has proposed the method in which the frequency is adjusted by forming a SiN film on an SiO2 film formed on the interdigital electrodes and thinning the thickness of the SiN film by a physical etching method or increasing the thickness by a sputtering method.
Further, International Publication Pamphlet No. WO 2005/093949 has proposed has proposed the frequency adjustment method by adjusting a thickness of second medium in a boundary wave device having inter-digital transducer electrodes disposed at boundary between the first and the second mediums.